Development of neuronal specificity in retinal ganglion cells of Xenopus

Abstract

Summary Retinal ganglion cells of Xenopus form topographically organized connections with the optic tectum between larval stages 35 and 40. However, the position at which each retinal ganglion cell connects in the tectum is already fully specified at larval stage 31, before visible differentiation of ganglion cells and before outgrowth of their axons. This was shown by 180-degree rotation of the eyecup of Xenopus larvae at stages 28–35. After metamorphosis of these animals, the projection from the rotated eye to the optic tectum was mapped electrophysiologically, and compared with the normal retinotectal map. Normal retinotectal connections were formed after rotation of the eyecup before stage 30. Eye rotation at stage 30 resulted in reversal of the retinotectal map in the AP axis, but not in the DV axis of the retina. Rotation of the eye after stage 30 resulted in total rotation of the retinotectal map. Specification of the central connections of retinal ganglion cells occurs over a period of about 10 hours, in two steps related to the relative positions of the ganglion cells in the AP and DV axes of the retina. The hypothesis is advanced of stepwise specification of progressively finer details of the pattern of neuronal connections; the initial steps occurring at an early stage of neuroblast differentiation before the outgrowth of neuronal processes.